Device for generating a digitally modulated test signal

ABSTRACT

The aim of the invention is to generate a digitally modulated test signal, which is generated in the form of a multitude of transmission channels from a digital modulation data stream according to a predetermined digital modulation standard, and which is fed as I and Q values to an I/Q modulator. To this end: a) a portion of the transmission channels is generated, in a modulation coder, directly from an internal or external modulation data stream according to the predetermined modulation standard; b) the I and Q values for at least one portion of the remaining channels are fed as a pre-calculated modulation data sequence of alimited length from a memory to the modulation coder, and; the I and Q values generated according to a) and b) are added and fed to the I/Q modulator.

BACKGROUND

1. Field of the Invention

The invention is based on and relates to an arrangement for generating adigitally modulated test signal that is generated in the form of a largenumber of transmission channels from a digital modulation data flowaccording to a predetermined digital modulation standard and is fed as Iand Q values to an I/Q modulator.

2. Description of the Related Art

Digitally modulated high-frequency or baseband test signals, which aregenerated in signal generators, are often required for measurementpurposes. An enormously wide variety of digital modulation processeshave now come into use and depending on these and the measurement tasks,signal generators of this kind operate by one of the following methodsof signal processing.

In the first method, a modulation data stream generated internally orexternally in a data source is converted into I and Q values by codingand mapping (set of rules which assigns I and Q values to eachmodulation symbol as a function of the particular complex type ofmodulation) in a modulation coder and is then fed to the downstream IQmodulator, whose output signal is then converted to the desired highfrequency (the Rohde & Schwarz company's SMIQ vector signal generator,data sheet PD757.4582 and extract from associated manual 1084.80004.03,pages 2.78 to 2.112). The internal data source used may for example be amemory from which the data is read out. It is also known for any desiredcomplex data sequences to be assembled from stored data by means of asignal processor, an example of such data sequences being so-called TDMAsignals such as are used for global mobile telephone systems (see thevarious digital modulation standards which are possible, as listed on p.8 of the data sheet for the SMIQ signal generator). The data sequencesthat are assembled internally or externally in this way can be processeddirectly in real time. They may however also be buffer-stored in amemory and only then fed to the IQ modulator.

A second method of signal processing comprises calculating the I and Qvalues and storing the sequence of I/Q values that has been calculatedin this way in w memory for I and Q and then reading the digital IQvalues out from this memory, converting them into analog signals andthen feeding them directly, in filtered form, to the IQ modulator (atwo-channel ARB generator, e.g. the Rohde & Schwarz company's AMIQmodulation generator, data sheet PD757.3970.12 and associated unitspecification 1110.3339.11, pages 4.1 to 4.14). This second method issuited above all to modulation standards where a large number ofindividual transmission channels are generated simultaneously, as hasnow become standard practice on many modern-day mobile telephonenetworks. Under the so-called CDMA standard, 64 transmission channelsare for example generated simultaneously by means of a so-called Walshcode (as described in, for example, “North American Cellular CDMA”,Hewlett-Packard Journal, December 1999, pages 90 to 97), while under theup-to-date W-CDMA method (see description in the AMIQ data sheet, page9) there are even up to 512 individual transmission channels generated,which are modulated simultaneously onto one or more carriers. Thissecond method does however have the disadvantage that it is not possibleto operate in real time, i.e. pre-calculated signals have to be used andexternal data supplied by the user cannot be made use of. Thepre-calculated items of data have to be stored in a memory at a limitedlength and can therefore only be assembled into a data sequence of anylength by being repeated a multiplicity of times in succession. Thereare therefore limits to how far measurements where the content of thedata is crucial can be made, such measurements being required forexample for so-called BER (bit error rate) measurement (see the AMIQdata sheet, page 5). For synchronization with data on higher layers(under the ISO layer model) as well or for decoding tests, it isnecessary for a lengthy data sequence to have the correct data contentor for data made available from outside to be processed in real time.

Hence, although signal generators that operate by the second method dogenerate complex signals with the correct spectrum and the correctsignal statistics, there is, because of the limited storage length, alimit to how far measurements where the correct content of the data iscrucial can be made.

Signal generators that operate by the first method on the other handare, it is true, suitable for making measurements where the correctcontent of the data is crucial, but they cannot be used for modulationstandards under which a large number of transmission channels are used,because the cost and complication required for this purpose would beunacceptable and the performance of normal computers would not be goodenough for it. Signal generators that operate by the first method aretherefore so far being used only for measuring a few, e.g. four,channels (e.g. for BER measurements, synchronization with data on higherlayers, decoding tests) and the remaining channels, such as theremaining 508 channels in the case of W-CDMA for example, are leftunused. Such measurements are thus not a true reflection of realitybecause the neighboring channels too have an effect on the measuredresult. Nor does adding noise as a substitute for the neighboringchannels give a true reflection of reality because noise does not givethe orthogonality between the channels that is required under thestandard and hence affects the reception characteristics and themeasured results are thus falsified.

Hence, although measurements on receivers can be made in real time withsignal generators that operate by the first method, the conditions arenot a true reflection of reality because only some of the channels arebusy and the remaining channels are missing.

SUMMARY

It is therefore an object of the invention to provide an arrangement forgenerating a digitally modulated test signal that is generated accordingto one of the usual digital modulation standards employing a largenumber of transmission channels but that can nevertheless be used tomake measurements where the correct content of the data is critical.

This object is achieved on the basis of an arrangement as detailed inthe preamble of the main claim by virtue of the characterizing featuresof the claim. Advantageous refinements can be seen from the subclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows a block diagram of an arrangement comprising amodulation coder that operates by the first method described above and amodulation coder that operates by the second method described above.

DESCRIPTION OF PREFERRED EMBODIMENT(S)

An arrangement according to the invention combines within it theadvantages of both the known methods mentioned above. For measurementtasks where the correct data content is crucial (BER measurements,synchronization with data on higher layers, decoding tests and thelike), it is true that only a proportion of the total number oftransmission channels available is used, but realistic data is alsoapplied to the neighboring channels, though it is only calculated datathat is read out of a memory in which the storage length is limited,this though being sufficient to produce a test signal that overall is atrue reflection of reality. Unfalsified measured results are obtained inthis way. The signals that are generated by the two different methodsare added synchronously and with the correct temporal correlation withone another, i.e. the clock signal with which the IQ values that aregenerated by the first method, preferably in real time, are fed to theIQ modulator is selected to be equal to the clock signal with which theIQ values obtained by calculation by the second method are read out ofthe memory. This ensures that the orthogonality that is required betweenthe individual channels is preserved without being adversely affected.

The data sequence for the actual measurement channels in the firstmethod can be fed in a known manner directly from an internal orexternal data source, and in the simplest case what is used as aninternal data source may for example once again be a memory, that isloaded with the pre-calculated modulation data sequence. From the datathat is read out, the I/Q signal proper is then generated in a knownmanner by coding and mapping in the modulation coder and is fed to theIQ modulator. The modulation data sequence in the first method may alsobe assembled in a known manner by means of a suitable signal processorfrom individual components that are either stored in a memory orcalculated in real time. In this way it is possible for a measurement tobe made in real time even on the basis of internal data. If the data forthe first method is stored internally in a memory, different respectivecycle lengths, in the form of prime numbers for example, may be selectedfor this memory and the second memory used for the method. This gives aconsiderably longer overall cycle (the least common multiple of theindividual characteristics) and hence even more realistic signals, i.e.the test signal is not repeated so frequently and is thus an even truerreflection of reality.

The invention will be explained in detail below by reference to adiagrammatic drawing and an embodiment.

The FIGURE shows a block circuit diagram of an arrangement according tothe invention, comprising a modulation coder that operates by the firstmethod described above and a modulation coder that operates by thesecond method described above. The modulation coder is for examplesimilar in construction to the Rohde & Schwartz company's SMIQ vectorsignal generator and the modulation generator plus its internal memoryis also of known design. The IQ values from the modulation generator,which are obtained by calculation, are fed via a delay unit to theadding stages of the modulation coder and the IQ values from themodulation coder are thus added to the IQ values from the modulationgenerator with the correct temporal correlation between them and arethen converted into analog form and are finally fed to the IQ modulatorproper, in which they are modulated onto the carrier signal proper. Thememories of the modulation generator and the modulation coder are drivenby a common clock generator and the IQ values are thus read outsynchronously to a clock signal.

1. A method for generating a digitally modulated test signal comprising:generating a first group of transmission channels comprising I and Qvalues, in a modulation coder, directly from a modulation data streamaccording to a predetermined standard; generating a second group oftransmission channels comprising I and Q values by repeating apre-calculated data sequence of limited length, said pre-calculated datasequence being recalled from a memory of a modulation coder; adding saidI and Q values of said first group and said second group; and feedingthe result to an I/Q modulator to generate said test signal.
 2. Themethod of claim 1 wherein said adding of said first group and saidsecond group takes places synchronously with a clock signal and with thecorrect temporal correlation between the I and Q values.
 3. The methodof claim 2 wherein the I and Q values of the first group arebuffer-stored in a memory having a different storage length from amemory that receives the I and Q values of the second group.
 4. Themethod of claim 3 wherein the I and Q values of the first group aregenerated in real time directly for said modulation data stream.
 5. Themethod of claim 4 wherein said modulation data stream is an internalmodulation data stream.
 6. The method of claim 4 wherein said modulationdata stream is an external modulation data stream.
 7. The method ofclaim 1 wherein the I and Q values of the first group are buffer-storedin a memory having a different storage length from a memory thatreceives the I and Q values of the second group.
 8. The method of claim7 wherein the I and Q values of the first group are generated in realtime directly for said modulation data stream.
 9. The method of claim 8wherein said modulation data stream is an internal modulation datastream.
 10. The method of claim 8 wherein said modulation data stream isan external modulation data stream.
 11. The method of claim 1 whereinthe I and Q values of the first group are generated in real timedirectly for said modulation data stream.
 12. The method of claim 11wherein said modulation data stream is an internal modulation datastream.
 13. The method of claim 11 wherein said modulation data streamis an external modulation data stream.
 14. The method of claim 1 whereinsaid modulation data stream is an internal modulation data stream. 15.The method of claim 1 wherein said modulation data stream is an externalmodulation data stream.